Water-based ink that uses water as a main solvent has outstanding features such as it possesses little or no fire hazard and it has little or no toxicity such as mutagenicity, unlike solvent ink. This makes water-based ink the mainstream ink for ink jet recording in usages other than industrial usages.
The properties needed for ink used in ink jet recording are, for example, as follows: (1) offering an image having high coloring property, high resolution, high density, and uniformity is created on a recording medium without bleeding, (2) clogging of nozzle tips caused by drying of ink does not occur and discharge stability is high, (3) the drying property of the ink on a recording medium is satisfactory, (4) the image has good fastness, and (5) the long-term storage stability is high.
Previously, dyes have been used as colorants of water-based ink for ink jet recording since dyes have high dissolution stability, cause less nozzle clogging, offer high coloring property, and thus enable high-quality image printing; however, images created with dyes have a problem in that the water resistance and lightfastness are poor.
In order to address this problem, more and more pigments are used as colorants instead of dyes. While pigment ink is expected to achieve high water resistance and lightfastness, the coloring property is poor compared to dyes and nozzle clogging caused by aggregation and sedimentation of pigments arises as a problem. Thus, various studies have been made on specific methods for dispersing microparticulated pigments in water-based media by using polymeric dispersants.
For example, a method has been proposed which includes preparing an aqueous solution by dissolving a water-soluble resin and an alkali component in water, adding a pigment to the aqueous solution followed by thorough stirring, and dispersing the mixture using a high-speed sand mill or the like having high dispersion efficiencies (for example, refer to Japanese Unexamined Patent Application Publication No. 2001-262038).
However, according to this method, it takes a long time for dispersing and the production efficiency has been low. Moreover, the dispersion stability of the pigment in the resulting water-based pigment dispersion liquid has been unsatisfactory.
Under these circumstances, a dispersing method has been proposed which includes a pretreatment step of preliminarily kneading a pigment and a polymeric dispersant (for example, refer to Japanese Unexamined Patent Application Publication No. 2003-226832). This has improved production efficiencies and has made the pigment finer. However, although pigment particles became finer, the dispersion stability of the pigment has been unsatisfactory and thus the storage stability of the ink remains to be improved. In particular, quinacridone type pigments used in producing magenta ink are hydrogen-bonding-type pigments that function as pigments through intermolecular hydrogen bonding and thus pigment particles are strongly aggregated and tend to remain undispersed by forming coarse particles in the ink. Moreover, the pigment particles which have been made finer by dispersion tend to undergo reaggregation. Thus, the dispersionstability has been particularly difficult to achieve.
To address this, a method for producing a water-based pigment dispersion liquid has been proposed in which a quinacridone type pigment, a phthalimidomethylated-quinacridone type compound or additionally a quinacridonesulfonic acid type compound, and an anionic-group-containing organic polymer compound having a glass transition temperature of −20° C. to 60° C. are used (for example, refer to Japanese Unexamined Patent Application Publication No. 2004-043791). This water-based dispersion liquid has good dispersibility and dispersion stability, the particle size does not increase much even when left at high temperature for a long time, and the increase in viscosity is suppressed such that the viscosity can be maintained in a low viscosity range, which makes the dispersion liquid suitable for use in water-based ink for ink jet recording.
However, all of the pigment dispersion processes described above give dispersions containing unsatisfactorily disintegrated pigment particles and coarse particles that are not dispersed sufficiently. Although introduction of the kneading step reduced the number of such coarse particles, removal of coarse particles by centrifugal treatment and filtering treatment is still needed in order for the dispersion to be actually used as water-based ink for ink jet recording. Although the amount of remaining coarse particles can be reduced by introduction of the centrifuging and filtering step, the pigment yield is decreased and the production efficiency is significantly degraded, posing many challenges on the productivity. In particular, use of quinacridone type pigments has made the problem more difficult to resolve.